Electric Double Layer (EDL) capacitors have much higher capacitance than conventional electrostatic and electrolytic capacitors—up to tens or hundreds of farads per gram of active electrode material. Consequently, EDL capacitors can be usefully employed as backup power sources in systems requiring uninterrupted electric power supply. For example, EDL capacitors are used for this purpose in computers and communication devices.
For example, some conventional EDL capacitor designs are disclosed in U.S. Pat. Nos. 4,313,084 and 4,562,511. Conventional EDL capacitors include two porous polarizable electrodes that are each coupled to a respective current collector. The electrodes are typically made of carbonaceous materials and are spaced apart by a porous dielectric separator. The electrodes, the current collectors and the separator are fixed in place by a casing, which also encloses some amount of a free volume. A liquid electrolyte, which can be either non-aqueous or aqueous, is retained in the pores of the electrodes, the separator and in the free volume.
A disadvantage of conventional EDL capacitors is that they have rather low specific energies. A conventional EDL capacitor employing a non-aqueous electrolyte will have a maximum specific energy of about 3 Wh/l and maximum voltage value in the range of 3 to 3.5 V. Non-aqueous electrolytes generally have low conductivity and, as a result, EDL capacitors employing them can only permit relatively low discharge and charge currents. Employing an aqueous electrolyte produces EDL capacitors having even lower specific energies (e.g., approximately 0.5 to 2 Wh/l) and lower maximum voltage values (e.g., approximately 0.9 V); and furthermore, the positive electrode of such a EDL capacitor is prone to corrosion or oxidation when the EDL capacitor is charged for long periods, as it may be in a backup power supply.
As an alternative, there are a number of EDL capacitors that are each made up of one polarizable electrode and one non-polarizable electrode. The non-polarizable electrode is the Positive Electrode (PE) and the polarizable electrode is the Negative Electrode (NE).
One example of an EDL capacitor having only one polarizable electrode is described in a published Japanese Application, No. 2-11008. The non-polarizable electrode is made of either lithium or a lithium alloy and a non-aqueous solution serves as the liquid electrolyte. This EDL capacitor has a higher specific energy than a conventional EDL capacitor. However, because the electrolyte is non-aqueous, this EDL capacitor can only handle/support relatively low discharge and charge currents; and, therefore, very low power density. This EDL device also has a relatively low cycleability. That is, it may only be used in a relatively low number (e.g., 100 to 200) of charge and discharge cycles before it is expected to fail.
Another example of an EDL capacitor with only one polarizable electrode is disclosed in PCT Patent Application WO 97/07518. The non-polarizable electrode is made of nickel oxide and an aqueous solution of alkaline metal carbonate or hydroxide serves as the liquid electrolyte. This EDL capacitor is considerably better than a conventional EDL capacitor with respect to both specific energy and maximum voltage. However, this EDL capacitor has a number of shortcomings. First, although the specific energy is high it is not high enough for some applications. Second, this EDL capacitor is expensive to produce since it requires large quantities of nickel oxide.
U.S. Pat. Nos. 6,195,252, 6,426,8862, 6,466,429, and 6,628,504 disclose EDL capacitors in which a non-polarizable electrode (i.e., the positive electrode) includes an active mass made up of lead dioxide (PbO2) and lead sulfate (PbSO4). The electrolyte in these EDL capacitors is contained only in the pores of the electrodes and the separator. There is no free electrolyte within the EDL capacitors since the components are pressed together by reinforcement plates. The pressure provided by the reinforcement plates is necessary to maintain a somewhat reliable contact between the polarizable electrode (i.e., the negative electrode) and the protective layer of the negative electrode current collector.
However, the pressure is not enough to guarantee reliable contact after a significant number of charge and discharge cycles. Volume changes of several tens of percents (up to 50%) are typical during cycling. After repeated volume changes the contact between the active mass and the rest of the positive electrode gradually becomes weaker and the active mass sheds from the rest of the positive electrode. Specifically, the bond between the active mass and the rest of the positive electrode weakens in the direction coinciding with the electrode plane; since, in this direction there is no compressive force between the active mass and the rest of the positive electrode.
Moreover, an active mass under high compression has low porosity, which, in turn, brings about a decrease in capacity. There are also other factors that cause the electrodes to have uneven thicknesses, which, in turn leads to uneven compression and varying pore structure across the surface of the electrodes. Consequently, the potential on the electrode surfaces and the electrolyte will be distributed non-uniformly; and, thus the capacitance is lowered and there will be a substantial self-discharge effect that is also detrimental to the performance of the EDL capacitor.